Thermal spray techniques are used to deposit wear resistant or thermally insulating coatings from metal and/or ceramic powders, on various components. For example, ceramic powders are thermally sprayed on the face of engine piston crowns and valves to deposit thermal barrier coatings on these components. In other instances, metal powders are thermally sprayed on various engine components to alter the thermal conductivity and/or wear characteristics of such components.
Metal coatings deposited by thermal spray techniques generally have a high oxygen content when compared to the oxygen content in the wrought metal. It is important to reduce the amount of oxygen present in the metal coating in order to improve the formability of the coating, to make the coating less brittle, and to improve corrosion resistance.
Various methods for reducing the oxygen content in thermally sprayed metal coatings are known to those skilled in the art. One such method is to thermally spray the metal powder in a chamber filled with an inert gas, such as nitrogen, for example. Another method is to use an inert gas shroud to protect the molten powder from oxidation during the thermal spray process.
One common problem encountered in the thermal spray process is the susceptibility of the sprayed metal powder to oxidation. This problem becomes more severe when one uses metal powders that have been prepared by water atomization methods. Commercially available water atomized metal powders are about half the cost of gas atomized metal powders and hence the use of gas atomized metal powders represents a waste of labor and resources. However, water atomized metal powders contain about five to ten times greater oxygen than gas atomized metal powders. Typically, water atomized metal powders contain about 10,000 ppm to about 20,000 ppm of oxygen by weight whereas gas atomized metal powders contain 100 ppm to 500 ppm oxygen by weight. Even water atomized metal powders that have been annealed contain about 1,000 ppm to 5,000 ppm oxygen by weight.
None of the heretofore mentioned thermal spray methods facilitate the lowering of oxygen content in the sprayed metal coating to ultra-low levels, such as equal to or less than 500 ppm, or 0.05% oxygen by weight. A technical article titled "Sprayforming by High-Power High-Velocity Plasma Spraying" by M. Scholl, P. Clayton, E. Elmore and J. Wooten, published in the proceedings of the Fourth National Thermal Spray Conference, Pittsburgh, Pa., U.S.A., May 4-10 1991, pages 281-288 further illustrates this problem. In that technical publication, the authors reported the problem of a six-fold increase in the oxygen content of the sprayed deposit as compared to the oxygen content in the metal wire.
A process for reducing the oxygen content in metal articles formed by powder metal pressing (PMP) is known to those skilled in the art. This process involves the addition of carbon to a metal powder prior to pressing. One drawback with this process is the requirement of an additional step of annealing. After pressing the powder metal into a desired shape, the pressed metal article must be annealed to reduce the oxides. This additional step of annealing represents a waste of time, labor and resources.
It has been desirable to have a method of depositing high quality metal coatings by thermal spray methods which result in the metal coating having a lower oxygen level as compared to the metal powder being sprayed, without requiring the additional step of annealing. It has further been desirable to have a metal mixture which can be thermally sprayable to form a metal coating having an ultra-low oxygen content without the requirement of annealing the coating. It has still further been desirable to have a thermally sprayed metal coating having low oxygen content after thermal spray deposition without requiring additional annealing. It has yet further been desirable to achieve comparably low levels of oxygen in a resultant metal coating thermally sprayed using gas or water atomized metal powders, without employing the labor intensive additional step of annealing the metal coating after thermal spray deposition.
The present invention is directed to overcome one or more problems of heretofore utilized methods for reducing oxygen content in metal coatings which are deposited by thermal spray techniques.